Implement basic I/O balancing code (for read/write) for multipath
personality. The code is based on RAID1 implementation.
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxx>
---
drivers/md/multipath.c | 70 ++++++++++++++++++++++++++++++++++++++---------
drivers/md/multipath.h | 1 +
2 files changed, 57 insertions(+), 14 deletions(-)
diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
index 3535c23af288..83c4f5105705 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -30,29 +30,58 @@
#define NR_RESERVED_BUFS 32
-
-static int multipath_map (multipath_conf_t *conf)
+/*
+ * This routine returns the disk from which the requested read should
+ * be done. There is a per-array 'next expected sequential IO' sector
+ * number - if this matches on the next IO then we use the last disk.
+ * There is also a per-disk 'last know head position' sector that is
+ * maintained from IRQ contexts, IO completion handlers update this
+ * position correctly. We pick the disk whose head is closest.
+ *
+ * Note that 'sector' argument is for original bio whereas 'head_position'
+ * is maintained for each rdev so we should take it into account when
+ * calculating the distance.
+ */
+static int multipath_map(multipath_conf_t *conf, sector_t sector)
{
int i, disks = conf->raid_disks;
-
- /*
- * Later we do read balancing on the read side
- * now we use the first available disk.
- */
+ int best_disk;
+ sector_t best_dist;
rcu_read_lock();
+retry:
+ best_disk = -1;
+ best_dist = MaxSector;
+
for (i = 0; i < disks; i++) {
+ int dist;
mdk_rdev_t *rdev = rcu_dereference(conf->multipaths[i].rdev);
+ sector_t this_sector = sector;
+
if (rdev && test_bit(In_sync, &rdev->flags)) {
- atomic_inc(&rdev->nr_pending);
- rcu_read_unlock();
- return i;
+ this_sector += rdev->data_offset;
+ dist = abs(this_sector - conf->multipaths[i].head_position);
+ if (dist < best_dist) {
+ best_dist = dist;
+ best_disk = i;
+ }
}
}
+
+ if (best_disk == -1) {
+ printk(KERN_ERR "multipath_map(): no more operational IO paths?\n");
+ } else {
+ mdk_rdev_t *rdev;
+
+ rdev = rcu_dereference(conf->multipaths[best_disk].rdev);
+ if (!rdev || !test_bit(In_sync, &rdev->flags))
+ goto retry;
+
+ atomic_inc(&rdev->nr_pending);
+ }
rcu_read_unlock();
- printk(KERN_ERR "multipath_map(): no more operational IO paths?\n");
- return (-1);
+ return best_disk;
}
static void multipath_reschedule_retry (struct multipath_bh *mp_bh)
@@ -82,6 +111,17 @@ static void multipath_end_bh_io (struct multipath_bh *mp_bh, int err)
mempool_free(mp_bh, conf->pool);
}
+/*
+ * Update disk head position estimator based on IRQ completion info.
+ */
+static inline void update_head_pos(int disk, struct multipath_bh *mp_bh)
+{
+ multipath_conf_t *conf = mp_bh->mddev->private;
+
+ conf->multipaths[disk].head_position =
+ mp_bh->bio.bi_sector + (mp_bh->bio.bi_size >> 9);
+}
+
static void multipath_end_request(struct bio *bio, int error)
{
int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
@@ -89,6 +129,8 @@ static void multipath_end_request(struct bio *bio, int error)
multipath_conf_t *conf = mp_bh->mddev->private;
mdk_rdev_t *rdev = conf->multipaths[mp_bh->path].rdev;
+ update_head_pos(mp_bh->path, mp_bh);
+
if (uptodate)
multipath_end_bh_io(mp_bh, 0);
else if (!(bio->bi_rw & REQ_RAHEAD)) {
@@ -122,7 +164,7 @@ static int multipath_make_request(mddev_t *mddev, struct bio * bio)
mp_bh->master_bio = bio;
mp_bh->mddev = mddev;
- mp_bh->path = multipath_map(conf);
+ mp_bh->path = multipath_map(conf, bio->bi_sector);
if (mp_bh->path < 0) {
bio_endio(bio, -EIO);
mempool_free(mp_bh, conf->pool);
@@ -356,7 +398,7 @@ static void multipathd (mddev_t *mddev)
bio = &mp_bh->bio;
bio->bi_sector = mp_bh->master_bio->bi_sector;
- if ((mp_bh->path = multipath_map (conf))<0) {
+ if ((mp_bh->path = multipath_map(conf, bio->bi_sector)) < 0) {
printk(KERN_ALERT "multipath: %s: unrecoverable IO read"
" error for block %llu\n",
bdevname(bio->bi_bdev,b),
diff --git a/drivers/md/multipath.h b/drivers/md/multipath.h
index 3c5a45eb5f8a..060fe2aabd97 100644
--- a/drivers/md/multipath.h
+++ b/drivers/md/multipath.h
@@ -3,6 +3,7 @@
struct multipath_info {
mdk_rdev_t *rdev;
+ sector_t head_position;
};
struct multipath_private_data {
--
1.7.5.2
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